1. Field of the Invention
The present invention, relates to an acoustic imaging system, more particularly, an acoustic imaging system for producing real time images of internal portions of an object both in sector scan mode and in linear scan mode.
The acoustic imaging system is utilized, for example, as an ultrasonic diagnostic apparatus for producing real time cross-sectional images of the human body, and a defect detecting apparatus for detecting material or structural defects of an object.
2. Description of the Related Art
In an acoustic imaging system, a plurality of transducer elements, i.e., ultrasonic vibrators are placed in contact with an object, and ultrasonic waves are generated in the transducers so as to form a beam penetrating the object. Reflected waves are converted into corresponding electrical signals in the transducers. A focus point is established by controlling the delay time of individual electrical signals so as to compensate for path differences between the focus point and the individual transducers and by superimposing the signals.
To obtain real time images of the object, dynamic focusing, i.e., frequent change of values of the delay time to move the focus point must be attained. If the change of the delay time is implemented by switching switch elements, spike noises are generated in every switching, so that the obtained images are contaminated by the spike noises.
Japanese Unexamined Patent Publication (Kokai) No. 56-112234 discloses an ultrasonic imaging system comprising two systems of tap-switching type phased arrays including a plurality of switch elements and delay lines having a plurality of taps. The two systems of phased arrays are alternatively used to avoid invasion of spike noises when switching. The imaging apparatus disclosed in the above publication requires two systems of expensive phased arrays, and since whole phased arrays having a long delay time are alternatively used, the frequency of the switching is limited to a certain level.
U.S. Pat. No. 4,140,022 discloses a sector scan type acoustic imaging system comprising a plurality of phase changing means coupled to the phase control means to effect the dynamic focusing. The phase changing means preferably includes a mixer, a source of waves of a given frequency, and means for setting the phase of the waves coupled from the source to the mixer.
Since the ultrasonic imaging system disclosed in the above publication includes mixers, oscillators, and phase shifters, the whole system becomes expensive and complex, and in particular, in a linear scan type imaging apparatus wherein many signals have to be processed, these shortcomings become serious problems.
Furthermore, since the dynamic focusing is attained only by controlling the phase of carrier signals included in the received signals, in a recently required probe having a large imaging area wherein phase variation more than three times as large as the wavelength of the ultrasonic signals is required in the dynamic focusing, deterioration of the ultrasonic beams occurs in the region where large phase variation is required in dynamic focusing. The reason why the deterioration of the ultrasonic beams occurs is that focusing becomes imperfect in that region, because a central frequency of the frequency distribution of the ultrasonic wave shifts toward the lower side because of attenuation in a higher frequency component in the living body. Since magnitudes of the frequency shifts are different in individual human bodies, the frequency shift cannot be adequately compensated for. It is also the reason why only the phase of the carrier signals is adjusted by the phase control, but envelopes of the signals are not adjusted by the phase control.
The deterioration of the ultrasonic beams might be avoided by performing complex processes wherein a single scan is divided into a plurality of steps in a depth direction, and dynamic focusing is carried out for the respective steps, with the results synthesized into a single scan line. However, these processes require a long processing time to obtain a cross-sectional image, and therefore, reality of time, which is an essential purpose of the dynamic focusing, is lost. In addition, division into a few steps in a depth direction is not enough to avoid the deterioration of the beams.
To realize the linear scan mode, parts of the vibrators have to be selected by multiplexers according to positions of the scan line as disclosed in U.S. Pat. Nos. 4,224,829 and 4,699,009. Nevertheless, the linear scan and the dynamic focusing could be simultaneously realized by N variable gain amplifiers for performing dynamic apodizing and an N:M switch matrix where N is a number of the vibrators and M is a number of input taps of a delay line as described with reference to drawings later, if the problems of the spike noises are to be resolved and if the numbers N and M are not so large. However, in recent years, imaging systems having high resolution have been required, as ultrasonic diagnostic systems have been widely used. In order to realize a higher resolution, the number of transducer elements N must be large. For example, an apparatus wherein N=2128 has been manufactured. Also, resolution of the delay time in the delay line must be refined to realize a high resolution. To refine the resolution of the delay time, the number of the input taps of the delay line M must be large. For example, M is required to be up to 128.
As mentioned above, the imaging system having a higher resolution requires the larger number of N and M, and therefore a switch matrix having a great many switch elements is required. For example, if N=128 and M=128, then the number of switch elements becomes 128.times.128=16,384. In order to realize this, if commercially available matrix switch IC's of 8:16 are used, (128.times.128)/(8.times.16)=128 of IC's are required, and it is not practical to use the same.